1. Field of the Invention
The present invention relates to a small brush-use DC motor.
2. Related Art
A conventional small brush-use DC motor is disclosed in Unexamined Japanese Utility Model Publication No. Sho. 64-54754, for example. The DC motor is constructed as shown in FIG. 14.
In FIG. 14, reference numeral 71 designates a stator case 71, shaped like a cup. The stator case 71 has a tubular portion 71b in the central part of the bottom thereof. The tubular portion 71b passes through the bottom of the stator case 71.
A first bearing 80 is fitted into and fixed to the tubular portion 71b. A side plate 73 is pressed to or press fitted to the open end (the top side thereof in the drawing) of the stator case 71. A brush holder 74 is fastened on the inner side of the side plate 73 by means of screws or resin molding.
A cap-like portion 74a is protruded upward (toward the side plate 73) in the central part of the brush holder 74. A second bearing 81 is fit into and fixed to the tubular portion 74a.
The first and second bearings 80 and 81 cooperate to rotatably support a rotary shaft 82 of an armature 76.
An armature core 84 with a winding 83 thereon and a commutator unit 85, both being fit to the rotary shaft 82, make up the armature 76. A ring-like magnet 72 is disposed in opposition to the peripheral outer surface of the armature core 84, with a predetermined gap therebetween. The ring-like magnet 72 is fastened to the peripheral inner surface of the cup-like stator case 71.
A contact part 75b, which is formed at the distal end of the brush 75, comes in contact with the peripheral surface of the commutator unit 85 in a slidable fashion. The base of the brush 75 is fastened to the brush holder 74. The brush 75 has a terminal 75a with a hole, not shown. The hole of the terminal 75a is fit into the tubular portion of the brush holder 74, and positioned at the location where the hole of the terminal is fitted and the tubular portion 74a is thermally deformed and welded.
A window hole 71a is formed in the stator case 71. The terminal 75a of the brush 75 passes through the window hole 71a to exterior. A collar 86 is fit to the lower part of the rotary shaft 82, which is located below the armature core 84.
The terminal 75a of the brush 75 is connected to a power source, not shown. The commutator unit 85, which is brought into press contact with the contact part 75b of the brush 75, is mounted thereon. The small DC motor is driven to rotate by changing the direction and phase of a current fed from the brush 75.
In the DC motor thus constructed, a clearance is present between the rotary shaft and the first and second bearings. The clearance possibly allows the related part, or the rotary shaft, to move in the radial direction. When the motor is driven, the play of the related part will create motor noise.
An attempt to reduce the play is one of the ways to reduce the motor noise. However, this attempt creates another problem. That is, when the play is reduced, the rotary shaft comes in area contact with the first and second bearings. The rotation resistance of the rotary shaft when it rotates being supported by the first and second bearings increases.
The small brush-use DC motor of the type in which the tubular-shaped commutator unit is used, generates low noise and has good motor performance. However, this type motor is disadvantageous in that because of the tubular shape of the commutator unit, the tubular commutator unit occupies a larger space expanding in the longitudinal direction.
In this type of the DC motor, the brush-to-brush spatial distance is narrower than the width (diameter) of the commutator unit. For this reason, in assembling the motor, the commutator unit must be inserted in the narrow brush-to-brush space. This makes the assembling work difficult.
A small brush-use DC motor of the type in which the commutator unit is of the disk type, is also known. This type of the DC motor, unlike the DC motor using the tubular commutator, is free of the difficulty of the assembling work, but it is disadvantageous in that the disk-like commutator unit occupies a larger space expanding in the radial direction than the tubular commutator unit.
In the motor-driven music box disclosed in Examined Japanese Utility Model Publication No. Hei. 1-21357, the rotary drum, the vibrating plates, the reduction gear train, and the DC motor are supported by a frame as the base board of the music box.
The gears of the reduction gear train are rotatably supported in a manner that the axes of the gears are inserted into indentations or bearings. The DC motor is laterally mounted on the frame such that the rotor shaft is in parallel with the frame, and the base plate of the motor is fastened to the frame by means of screws.
As described above, to mount the DC motor on the frame, the motor base plate must be fastened to the frame by means of screws, while to assemble the reduction gears into the frame since, the gears are merely put down into the indentations or bearings. In this respect, the DC motor mounting work is more difficult than the gear assembling work.